1971 Merc 800 to 1250 swap

[TomSawyer]

I have decided to swap from a 80 to a 125 both the same vintage. As far as I can tell it looks like everything should just bolt up. Is this really the case or are there things I should be aware of? How about the tilt and trim, just take off the lines at the pump and leave attatched to old motor and vice versa? fuel line connector? how about the bolt holes on the motor mount, are they the same? Thanks in advance for any info. Oh yeah this is on a 1971 Tom Sayer lark. Its a 16ft deep v runabout. The tag says its rated for 125hp. With the 80hp I can top out at about 28mph (8500' elevation) and with 4 adults on board the two rear people need to come forward to get it to plane reasonably quick. Any speculation as to what the added 45 hp will do?

 

[Faztbullet]

Re: 1971 Merc 800 to 1250 swap

The last year of the 125hp was 1969 but everything should be a direct swap as mounting pattern, power trim, cables, fuel connector are the same. The 125 is about 40-50 lbs heavier than the 80, so your got more stern deeper in the water. Make sure you "prop" it for max rpm (4800-5400rpm) or it wont live. Propped correctly you should be in the 35-40 mph area....

 

[TomSawyer]

Re: 1971 Merc 800 to 1250 swap

Thanks for the info. I noticed on oldmercs.com about the year. Should be a pretty straight forward job since everything is universal. I will definately prop it with a tach. 35-40 is what I am hoping for but more importantly it should hop out of the water a little better. The rear of the boat is higher out of the water than the bow so I believe it could use a little more weight in the aft. I will hopefully be ready for the lake before the 4th.

 

[TomSawyer]

Re: 1971 Merc 800 to 1250 swap

Got the 1250 ss bolted on today and it is truely plug and play!! Sprayed fuel in the carbs and it fired right up. I need to replace the water pump and a fuel line and tape some wires....Ill be on the water tomorrow afternoon!View attachment 100707